National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia:

G. N. Elmanov, Assistant Professor, e-mail: GNElmanov@mephi.ru
E. A. Ivanitskaya, Post-Graduate Student
E. S. Zharikov, Post-Graduate Student

The subject of research includes the structure and phase transformations during a multistage crystallization of amorphous alloys with composition of Ni_71.5Cr_6.8Fe_2.7B_11.9Si_7.1 and Ni_63.4Cr_7.4Fe_4.3Мn_0.8B_15.6Si_8.5 alloys (AWS BNi-2, according to American Welding Society). Amorphous alloys were obtained in the form of tapes (0.04–0.05 mm thickness) by rapid solidification (rapid quenching) techniques. The changes in structure, phase composition and topology of amorphous alloys, surface were investigated after different crystallization stages at the temperature range of 450–650 ^oС, using differential thermal analysis, X-ray diffraction analysis, atomic force microscopy, scanning electron microscopy and energy-dispersive X-ray microanalysis. The sequence of crystalline phases, formation was defined together with their crystal structure, peculiarities of microstructure and nature of chemical elements, redistribution during crystallization of these amorphous alloys. According to this, the surface topology changes adequately reflect the process, occurring during crystallization. The α-Ni solid solution crystals (80–100 nm diameter) are formed in amorphous matrix at initial crystallization stage at the temperature range of 450–500 ^oС. The biggest part of boron in this solution is displaced on crystal boundaries and amorphous phase. At the second stage (500–550 ^oC), amorphous phase is crystallized with formation of fine metastable τ-phase with М₂₃B₆ crystal structure, which covers the entire surface of the sample with gradual grow. In the third crystallization stage (more than 550–600 ^oC), τ-phase is decomposed simultaneously into boride with Ni₃B-type structure and lowtemperature modification of β₁-Ni₃Si silicide. The first and second phases are characterized by low content of silicon and boron, respectively. There was made an offer to use a decomposition mechanism of metastable τ-phase, formed at intermediate crystallization stage. When temperatures are close to solidus, monoboride separation (Cr, Ni, Fe)B with high chromium content, which amount is increased with increasing of annealing temperature, occurs in alloys under study. Monoboride was observed by scanning electron microscopy, but not by X-ray diffraction method.

Authors express their gratitude to P. S. Dzhumaev and V. I. Skrytnyy for the carried out X-ray microanalysis and X-ray diffraction analysis of the samples.
This work was carried out as a part of the Center “Nuclear Systems and Materials” with the government support of the “Program to enhance the competitiveness of National Research Nuclear University MEPhI” (agreement with the Ministry of Education and Science of the Russian Federation (August 27, 2013; No. 02.a03.21.0005).

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